1. Field of the Invention
The present invention relates a process for the preparation of a biaxially drawn polyamide film. More particularly, it relates to an improvement in the process for the preparation of a biaxially drawn polyamide film comprising first drawing a polyamide film in the longitudinal direction and then drawing the film in the lateral direction while holding the film by a tenter method, whereby a biaxially drawn polyamide film having an excellent flatness is obtained.
2. Description of the Related Art
A biaxially drawn polyamide film has excellent toughness, heat resistance, cold resistance, transparency, printability, and chemical resistance and is characterized in that almost no pinholes are formed, and thus this film is widely used as a base film for packaging foods and the like. For example, for the formation of a food-packaging pouch, a base film is ordinarily subjected to printing, lamination, forming into a bag, food filling, and then heat sealing. In this case, the flatness of the base film has a great influence on the processing speed and yield at the steps of printing, lamination, and forming into a bag. Namely, if a portion of the base film is slack, a pitch shear is generated at the printing step or wrinkling is caused at the lamination or bag-forming step, and it is impossible to increase the processing speed. Accordingly, it is eagerly desired to improve the levelness in the base film, especially to moderate the "uneven tightening phenomenon".
By the term "uneven tightening phenomenon" is meant a phenomenon in which a part of the film in the lateral direction is shorter than the other part with respect to the longitudinal direction of the film. This phenomenon is confirmed by the difference of elongation when a certain tension is imposed on a biaxially drawn film and this tension then removed.
This "uneven tightening phenomenon" will now be described with reference to FIG. 1. In FIG. 1, a film having been cut in the longitudinal direction into a length (for example, 3 meters) larger than the distance (for example, 2 meters) between base lines 2 and 3 is placed on a flat table 1, and one end of the film is secured to the table by an adhesive tape 4. A certain load, for example, a load of 20 grams per cm of the width, is imposed on the other end of the film to stretch the film, and a bench mark 6 corresponding to the base line 3 is drawn on the film. Then the load is removed, and cut lines 5 are formed on the film in the longitudinal direction to cut the film into strips having a constant width (for example, 3 cm). Then a certain load, for example, a load of 60 grams, is imposed on the free end of each strip. When the base line 3 on the table 1 is compared with the bench mark 6 on th strip, a pitch shear is observed between them. It is considered that this phenomenon is due to the fact that the thickness of the film is not uniform in the lateral direction and the flatness is poor.
More specifically, in the production of a biaxially drawn polyamide film by a sequential biaxial drawing method, because of the influence of the tenter clip holding both side ends of the film in the lateral direction and the influence of the necking-occurring position at the lateral drawing, it is very difficult to obtain a film having excellent flatness and uniform thickness over the entire width of the film.
If the film is thin at specific portions, at the heat-setting and cooling steps subsequent to the lateral drawing, these portions undergo crystallization, relaxation or strain different from that undergone by the other portions. It is considered that this results in the occurrence of the uneven tightening phenomenon.
Various proposals have been made to prepare a drawn film having excellent flatness while moderating the uneven tightening phenomenon in the drawn film. For example, Japanese Unexamined Patent Publication No. 52-126476 proposes a process for improving the flatness of a film by imposing a high tension on the film between the steps of the longitudinal drawing and lateral drawing of the film or after the heat-setting step subsequent to the lateral drawing. However, if only a high tension is imposed on a film according to this process, the effect of improving the flatness is not sufficient. A satisfactory improvement is attained if this application of a high tension is combined with heat relaxation conducted after application of a high tension. Therefore, the process is economically disadvantageous in that a heat-relaxing apparatus is necessary.
Japanese Unexamined Patent Publication No. 58-55221 discloses a process for improving the flatness of a biaxially drawn and heat-set film by treating the film in steam maintained at a temperature of 90.degree. to 150.degree. C. under a certain tension. However, this process has a problem in that since an apparatus for the steam treatment of the film and the step of drying the film after the steam treatment are necessary, the manufacturing cost is high.